When an alternate key is defined with compression of duplicates, only the first duplicate key is contained in the file. The rest are not stored, but information is stored to enable correct recreation of the keys.
For example, suppose you write a record with an alternate key value "ABC". If you have enabled compression of duplicate keys, and you write another record with the same key value, the file handler does not physically store the duplicate key value in the index file. However, the record is still available along the alternate key path.